1. Field of the Invention
The present invention relates to a method and a circuit arrangement for field strength determination and to an integrated circuit.
2. Description of the Background Art
In remote keyless entry systems for motor vehicles, an unlocking of the vehicle occurs not with a mechanical key, but with an electronic key medium or an electronic key, for example, in the shape of a molded article, on which actuation elements are arranged. One or more integrated circuits, which realize the function of the electronic key medium or of the electronic key, are arranged on or in the key medium.
A so-called base station, which communicates or exchanges data with the electronic key in a wireless manner, is placed in the motor vehicle as a counterpart to the electronic key.
To unlock or lock the motor vehicle, a user actuates an associated control element on the electronic key, as a result of which a data transmission is initiated between the electronic key or its integrated circuit and the base station. If the information transmitted between the base station and the electronic key corresponds to a stipulated protocol and has the expected content, the motor vehicle is unlocked with the aid of the base station.
In passive entry/passive go systems or passive entry go (PEG) systems, actuation of the key to lock or unlock the motor vehicle is no longer necessary. The user of the motor vehicle must only keep an electronic key medium with him, for example, in form of a card.
When the user operates a door handle of the motor vehicle, this is detected in the motor vehicle and reported to the base station. The base station thereupon transmits a low-frequency carrier signal, for example, with a frequency of 125 kHz, to the electronic key medium. In addition, data may also be transmitted to the electronic key medium by the low-frequency carrier signal.
To receive the low-frequency carrier signal, the key medium has an antenna circuit with an antenna, for example, in the form of a coil, whereby the antenna circuit generates an output value, for example, a voltage, which is a function of the field strength and a function of characteristic parameters of the antenna circuit.
An exemplary circuit arrangement for obtaining field strength information or for determining the field strength is described in German Patent Application DE 101 59 551 A1, which corresponds to U.S. Pat. No. 6,922,553, and which is incorporated herein by reference. The output value is used to determine the distance of the key medium from the base station or from one or more transmitting antennas.
If more than one transmitting antenna is provided at different positions, for example, in a front area and in a back area, of the motor vehicle, the position of the user or the key medium relative to the motor vehicle can also be determined by so-called triangulation based on two determined distances from the particular antennas.
To determine the field strength independent of an orientation or position of the key medium relative to the transmitting antenna of the base station, devices are known in which three antenna coils, each perpendicular to one another, are provided. The specific field strengths of the antenna coils are vectorially superposed to calculate a resulting field strength.
When the distance between the antenna of the base station and the antenna of the key medium antenna circuit has been determined, it is then verified whether the determined distance lies within a permitted tolerance range. If this is the case, access to the motor vehicle is made possible by unlocking all or only certain locks. The unlocking of only certain locks can be made dependent on the position of the user relative to the motor vehicle.
If the user then enters the motor vehicle and operates a start button to start the engine, a distance or position determination is performed in the key medium also based on a field strength measurement. If the position determination indicates that the user is in the required position for starting the engine, the engine is started.
Apart from the access control and the engine start, there are numerous other application scenarios in which a distance measurement is made, for example, when the user climbs out of the motor vehicle and moves away from it.
It becomes clear from the above statements that there are high requirements for the distance or position measurement based on field strength determination of the carrier signal in the key medium.
As stated above, for the distance measurement an output value is generated by the antenna circuit, for example, an output voltage, which is a function of the field strength and a function of the characteristic parameters of the antenna circuit. The characteristic parameters of a particular antenna circuit represent the tolerances, production variations, and other specific properties of the components of the particular antenna circuit.
Because each antenna circuit has its specific characteristic parameters, which can deviate considerably from one another in practice, different antenna circuit output voltages result in different antenna circuits at identical field strength. If, for example, an atypical table is provided in the particular key medium for distance calculation, in which an assignment of the output voltage to field strength is stored, this can lead to obvious errors in the distance calculation.
In order to take this problem into account, a laborious calibration of the key media or the antenna circuits usually takes place during a manufacturing process. For this purpose, for example, a known reference field strength can be predefined from outside by a calibration station, which is used to generate a calibration value in the particular key medium. This method is very laborious and takes into account only the parameter situation during the calibration process. A change in the characteristic parameters by long-term effects, temperature, and variable operating voltage cannot be detected by calibration during the manufacturing process; i.e., the distance measurement becomes accordingly poorer with such long-term effects.
Another major problem is that the antenna circuit or its antenna is more greatly dampened depending on metallic objects in its environment, for example, a bunch of keys; as a result, its characteristic parameters are also substantially changed depending on the situation. These effects as well cannot be detected by calibration during the manufacturing process.